Effective February 15, 2010, the expiration notification process begins:

• Windows 7 RC users receive an expiration notification in the task bar one time per day.
• Windows 7 RC users are presented with the Notification Wizard one time every four hours and then one time every hour.

• Expiration notifications in the task bar continue.
• Computers that are running Windows 7 RC restart every two hours.
  Important When the computer restarts, your work will not be saved.

• Windows starts to a black desktop.
• Windows 7 RC users are presented with a Windows Activation screen. The screen states that the Windows that you are running is not genuine.
• Computers that are running Windows 7 RC restart every two hours.

See http://support.microsoft.com/kb/971767 for more information.